The present invention relates to aircraft gas turbine engines having a thrust augmentation system for selectively providing increased thrust output for limited times. More particularly, the present invention relates to a fuel control system for a gas turbine engine having a thrust augmentor, and in which an augmentor fuel pump provides the usual augmentor fuel supply function and also provides pressurized fuel for actuation of several hydraulic actuators for controlling engine operation. The augmentor fuel control system can serve as a backup fuel system in the event the engine""s main fuel pump becomes inoperable. Because the augmentor fuel pump can provide fuel both for hydraulic pressure demands and also for augmentor fuel demands at most flight conditions, the main and augmentor fuel systems are, in effect, dual redundant to maintain engine operation over a wide range of engine operating conditions.
Aircraft gas turbine engines sometimes include a thrust augmentation system for providing increased thrust for particular portions of an aircraft flight regime. Such increased thrust can advantageously be utilized at takeoff, and at other times when high-speed dash capability is desired to respond to particular exigencies during flight. Typically, the thrust provided by an engine that includes a thrust augmentation system can be of the order of about 150% of the thrust of the main engine when it is not in the augmented thrust mode.
Because of the magnitude of the increased fuel flow demanded during augmented thrust operation, the engine main fuel pump for the core engine normally does not have the volumetric capacity to provide the required increased fuel flow for full flight range augmented operation. Accordingly, an additional, augmentor fuel pump is provided to furnish the required additional fuel flow to allow operation of the augmentor over the full flight range. Typically, the augmentor fuel pump is a centrifugal pump. And because the augmentor is either in an on condition or an off condition, the augmentor fuel pump can be readily sized to provide the desired fuel flow when augmentation is demanded.
Aircraft gas turbine engines having a thrust augmentation system which is generally positioned downstream of the main or core engine, require the provision of a variable area exhaust nozzle in order to avoid the imposition of an excessive back pressure on either the fan or the main engine when the augmentor is in operation. Unless the exhaust nozzle area is increased when augmentation is in effect, the increased velocity imparted to the exhaust gas by the augmentor can cause an undesirable pressure build-up within the engine unless pressure relief is provided by increasing the exhaust nozzle area. Such increased back pressure can cause undesired engine operation, possibly even fan or compressor stall if the back pressure is of a sufficiently high magnitude.
The incorporation of a variable area exhaust nozzle thus requires that there be provision for a nozzle actuation system to enable the exhaust nozzle area to be changed when required during augmentor operation. Typical variable area exhaust nozzles have a series of interleaved panels that define a flow path of circular cross section. The panels are slidable relative to each other in a circumferential direction to allow the nozzle area to be enlarged or reduced, as dictated by the engine operating conditions. When a converging-diverging exhaust nozzle is employed, the throat area and the outlet area of the nozzle can be linked together mechanically in a predetermined relationship, or they can be separately actuated. Additionally, if desired a converging-diverging exhaust nozzle can also be angularly displaceable to provide vectored thrust is a direction that is at an acute angle relative to the engine longitudinal centerline.
Actuation of the leaves of the exhaust nozzle to change the nozzle area is generally effected by a separate, variable displacement hydraulic pump that provides pressurized fluid, such as engine lube oil, or the like, to several piston-cylinder-type actuators that are circumferentially arranged about the outer shell of the augmentor. The provision of such an additional pump, along with the attendant pump drive system, hydraulic conduits, hydraulic fluid cooling apparatus, and the like, adds additional weight and cost to the engine, each of which is, of course, undesirable in an aircraft engine. And because relatively large forces are required to actuate the exhaust nozzle leaves to thereby change the nozzle area, the large actuation forces are generally provided by utilizing high hydraulic pressures in order to minimize the size and the weight of the nozzle actuators.
It is therefore desirable to provide an aircraft gas turbine engine having a thrust augmentation system in which operation of the exhaust nozzle actuators can be effected without the additional weight of a separate hydraulic pump and without the weight and space occupied by its associated additional piping, valves, and other hardware.
Briefly stated, in accordance with one aspect of the present invention, a thrust augmentation system for an aircraft gas turbine engine is provided in which the augmentor fuel system is configured to provide pressurized fuel to operate the exhaust nozzle actuators and also provides a selectable ability to supply main burner fuel. The augmentor fuel system includes an augmentor fuel pump having the fuel pump inlet in communication with the source of fuel and having a fuel pump outlet. An augmentor fuel distributor is connected with the augmentor fuel pump outlet for receiving and distributing pressurized fuel to the thrust augmentor. A pressurized-fuel-operated actuator is connected with the augmentor fuel pump outlet for receiving pressurized fuel and for operating an actuatable position control element. A flow control valve is positioned between the augmentor fuel pump and the actuator for controlling the pressure of and the rate of fuel flow from the pump to the actuator. Augmentor fuel flow is controlled by an augmentor fuel control valve connected with the augmentor fuel pump and with the flow control valve for regulating delivery pressure and flow rate of fuel from the augmentor fuel pump in response to engine operating parameters and engine output demand parameters.
In accordance with another aspect if the present invention, a method is provided for controlling exhaust nozzle area by utilizing pressurized fuel from the augmentor fuel pump.